The present invention relates to compositions used both as mud sweeps and spacers to facilitate oil field production operations.
Various "drilling fluids" have been developed to facilitate oil field production operations. Certain drilling fluids are circulated through the bit, an integral part of rotary drilling, to carry cuttings from the bit. Drilling fluids wherein the fluid phase is oil, water or a combination thereof, are known to those skilled in the art as "drilling mud" or, simply, "mud." In addition to functioning to bring cuttings to the surface, mud lubricates and cools the drill stem and bit, deposits a mud cake on the wall of the borehole to prevent loss of fluids to the formation, and protects against blowouts by holding back subsurface pressures. Although mud was originally a suspension of earth solids (e.g., clays) in water, the mud used in modern drilling is a complex, multiphase mixture of liquids, reactive solids, and inert solids. The liquid phase may be crude oil, diesel oil, or water and may contain one or more conditioners.
Yet another aspect of oil field production upon which the present invention has an impact is the aspect of "cementing." Cement is a powder consisting of alumina, silica, lime and other substances which hardens when mixed with water. Cementing, in the oil field production area, is the process of pumping a cementitious slurry into a wellbore to perform various functions. Those functions include supporting casing by bonding the casing to the walls of the wellbore, isolating different zones in the well, protecting the pipe from corrosive fluids, protecting fresh water sands, and sealing perforations in casing. Common operations, involving cement, include "plug backing" and "squeeze cementing." To "plug back" is to fill part of a wellbore with cement, e.g., for side-tracking; that is, for drilling past an obstacle which has become permanently lodged in the hole. "Squeeze cementing" is the process of forcing cementing material under pressure into a specific portion of a well, such as openings, fractures or permeable zones.
Interaction of different types of mud, as well as interaction of mud and cement, can cause problems in the oil well environment. For example, mixing of water base muds and oil base muds can have a deleterious effect on the properties of each. Certain mud components have been known to become mixed with cement slurry during displacement processes, which has an adverse effect on the properties of the cement. Slaked cement contains about 62.5 percent of calcium hydroxide, which is a major source of trouble when cement contaminates drilling fluids. In addition to the above, outside factors, such as bromides and saltwater, can adversely affect mud and/or cement, especially water base mud.
Interaction of different types of mud and/or mud cement can occur during several oil field operations. Mud must, for example, be swept out of a well prior to primary cementing. If not completely swept away, and the combination of mud and cement slurry is incompatible, stringers of gelled mud fingering through the cement or adhering to the casing may result. This can adversely affect bonding of the cement. In any event, "flushing" with mud sweep prior to cementing is well appreciated by those skilled in the art as helping to control fluid loss and alleviating lost circulation during cementing. These later advantages result from the fact that mud sweeps, in a stiff gel form, seal off lost circulation zones by blocking flow channels and fractures. This causes higher fill-up efficiency and allows a higher top of cement. Additionally, downhold fluid loss is minimized. In general, a mud sweep should prevent cement flocculation, increase mud viscosity, and enable turbulent flow regimes at low pump rates.
Spacers are presweep fluids that can be weighted or densified to control hydrostatic pressure. Both viscous spacers which operate in the plug flow regime and turbulent spacers, which can combine high displacement rates with the ability to control densities, are known.
In the past, spacer fluids could not be successfully used as mud sweeps because they were not designed to protect soft shales. In addition, they did not contain the detergents necessary to remove sufficient oil and hydrocarbons from inside the casing to meet environmental standards.
U.S. Pat. No. 4,566,977 to Hatfield and entitled "Non-Aqueous Slurries Used as Thickeners" describes a non-aqueous slurry for use as a viscosifier in oil field drilling. The described slurry comprises a water-soluble cellulose ether polymer, a water-insoluble liquid hydrocarbon, a non-ionic surfactant with a hydrophilic-lipophilic balance of 7 to 14, and an organo-modified clay.
U.S. Pat. No. 4,670,164 to Watson et al. and entitled "Liquid Polymer Containing Compositions For Thickening Aqueous Systems" describes a composition useful as a thickening agent for aqueous systems used in well drilling and treatment. The described composition contains hydroxyethyl cellulose (HEC), a liquid hydrocarbon which does not cause significant swelling of HEC, and an organophilic clay. In addition, the composition may also contain an optional liquid, low molecular weight polar organic compound (e.g. lower alcohols or ketones) and a degellant to control the viscosity of the composition (e.g., fatty acids, or polyalkylene glycols).
U.S. Pat. No. 4,717,488 to Seheult et al. and entitled "Spacer Fluid" describes a slurry that can be used to prepare a spacer fluid for oil field use. The described spacer fluid consist of heteropolysaccaride S-130 (i.e., Biozan.TM. manufactured by Merck & Co., Inc.), an organophilic clay, a hydrocarbon solvent, an ethoxylated nonylphenol surfactant, and an optional lower alkanol.
The three patents mentioned above (i.e., U.S. Pat. Nos. 4,566,977; 4,670,164; and 4,717,488) describe compositions useful in oil field applications. However, not one of the described compositions is as advantageous in facilitating oil field production operations as the present invention. For example, Watson et al.'s composition does not contain a surfactant, which is essential to sufficiently clean the well casing of oil and hydrocarbons in order to meet environmental standards.
Furthermore, not one of the patented compositions described above contains potassium chloride. The inclusion of potassium chloride significantly improves the stability of the composition and prevents the hydration of "fines" (i.e., smaller clay or shale particles) and the consequent thickening of the composition. In addition, preventing shale hydration operates to protect soft shales and facilitates hole cleaning in severely washed out holes.
In fact, despite numerous efforts to develop an optimal composition that can serve as both a mud sweep and a spacer, no composition to date has been completely successful.
Based on the foregoing, a need has arisen for a composition that can serve as both a spacer and a mud sweep. Such a composition must be capable of separating cement from drilling fluids, separating water base muds from bromides, separating oil base muds from water base muds, and lessening the chance of contamination during plugging and squeezing operations. Important properties of such mud sweeps and spacers are high viscosity (even at high temperatures), excellent suspension (even at high temperatures), thickening efficiency in many different waters, high shear thinning and stability to salts, pH, and shear. Mud sweeps must also be effective in facilitating hole cleaning (even when the hole is severely washed out) and cement bonding (which improves chances for a successful cement job).